| English | Gas pressure |
|---|---|
| Bahasa Melayu | Tekanan gas |
| 中文 | 气体压强 |
Definition
The pressure produced by the collisions of gas particles with the walls of their container.
What you need to know
Gas pressure arises from the continuous collisions of fast-moving gas molecules with the walls of their container, as described by the kinetic theory of matter. Each collision exerts a tiny force on the wall, and the combined effect of countless collisions per second over the wall's area produces a measurable pressure.
Gas pressure is commonly measured using a manometer or a Bourdon gauge. A manometer is a U-shaped tube containing a liquid, usually mercury or water; the difference in liquid levels between the two sides, h, indicates a pressure difference of hρg, where ρ is the liquid's density in kilograms per cubic metre (kg m⁻³) and g is the gravitational field strength in metres per second squared (m s⁻²).
Depending on how the manometer is connected, the gas pressure equals atmospheric pressure plus hρg, or atmospheric pressure minus hρg, so the direction of the liquid-level difference must always be checked carefully to decide whether to add or subtract this term. A Bourdon gauge instead uses a curved metal tube that straightens slightly under pressure, giving a direct pointer reading.
Worked example
A gas supply is connected to a mercury manometer open to the atmosphere. The mercury level on the side connected to the gas is 8 cm lower than the level on the open side, showing that the gas pressure is greater than atmospheric pressure.
Take atmospheric pressure as 101 000 Pa, mercury density ρ = 13 600 kg m⁻³, and g = 9.81 m s⁻².
First convert the height difference into metres: h = 8 cm = 0.08 m.
The pressure difference is hρg = 0.08 m × 13 600 kg m⁻³ × 9.81 m s⁻² = 10 674.9 Pa, which can be rounded to about 10 700 Pa.
Since the gas side is lower, the gas pressure exceeds atmospheric pressure: gas pressure = 101 000 Pa + 10 700 Pa = 111 700 Pa. Every quantity used, height in metres, density in kilograms per cubic metre, and pressure in pascals, keeps its correct SI unit throughout the working.
How it is examined
In Paper 1, objective questions often ask candidates to calculate gas pressure from a manometer reading, or to identify whether gas pressure is greater or less than atmospheric pressure from a diagram showing the mercury levels. In Paper 2, structured questions typically ask candidates to explain the origin of gas pressure using the kinetic theory of matter, describe how a manometer is used to measure gas pressure, and calculate gas pressure by adding or subtracting hρg from atmospheric pressure as appropriate.
In Paper 3, practical tasks may ask candidates to determine gas pressure from a set-up diagram or measured mercury level difference, and to compare readings from a manometer and a Bourdon gauge.
A common mistake is adding hρg when it should be subtracted, or forgetting to identify which side of the manometer is connected to the gas supply before deciding on the sign.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)